Near-far problem is one of the most critical issues in wireless communication networks, significantly limiting network capacity. Received power of a transmitter's signal located far from a base station is much less than that of a nearer transmitter due to increased propagation losses. Without precautions, the adjacent channel interference (ACI) due to nearer transmitters can severely diminish the signal to interference plus noise ratio (SENR) of the far node's received signal in the uplink (UL) for frequency division multiple accessing (FDNIA) systems, resulting in the far transmitter's signal being undetectable at the base station. Orthogonal frequency division multiplexing (OFDM) is particularly sensitive to this issue. Conventional OFDM transmission emits incontrovertible energy in the out-of-band (OOB), whereas convention reception collects energy from OOB, due to the sine response of the rectangular pulse shape.
Many measures have been proposed to increase the far transmitter's capacity, such as power control and strict timing and frequency synchronization across links. However, these measures limit network capacity and the flexibility of the system. Power control limits the power transmitted by nearer transmitters in an effort to reduce interference, preventing transmitters with higher received powers from communicating at rates they would have otherwise achieved. The strict synchronization demands require the nodes to continuously track the synchronization signals and precisely adjust the transmit timing and frequency, accordingly. This continuous synchronization increases user equipment (UE) power consumption, and the added device complexity and precision requirements increase LTE costs. Furthermore, synchronicity imposes the same waveform with the same parameters to be used by all links in the network, which in the case of OFDM, is the same subcarrier spacing and cyclic prefix (CP) duration. Newer cellular communication generations are planned to allow waveforms with different parameters that are optimal for the link requirements, referred to as numerologies, in adjacent bands. For example, while low power Internet of Things (IoT) devices require smaller subcarrier spacings to converse battery, vehicular communications require higher subcarrier spacing and shorter symbol durations to keep the communication running in high Doppler spreads caused by higher speeds. Such asynchronous transmission is inherently non-orthogonal and interference is unavoidable.
Windowing of OFDM signals is a well-studied interference management technique in the waveform domain that has garnered attention due to its low computational complexity. Windowing can be performed independently at the transmitter to reduce OOB emission, or at the receiver to reduce interference caused by communication taking place in adjacent channels, commonly referred to as adjacent channel interference (ACI). Techniques have been proposed utilizing different window functions for each subcarrier at the transmitter and receiver and derived window functions for each subcarrier that maximizes the spectral localization and interference rejection.
Another critical problem that affects communication systems that use the OFDM waveform is its peak-to-average power ratio (PAPR), or the crest ratio. The PAPR is defined as the ratio of the peak power of the analog waveform to its average power. Before the low-power analog waveform at the output of the digital-to-analog converter is fed to the output of the transmitter; which can be an antenna in a wireless communication system, or a fiber-optical, co-axial or telephone wire or another medium in a wired communication system; it is fed to a power amplifier for amplification of the signal. The simplified relationship between the output voltage and the input voltage of modern power amplifiers assumes two regions; the linear region where the gain of the amplifier is linear if the input voltage is less than the saturation voltage followed by the saturation region for higher voltages. In the saturation region, the output voltage is the maximum output voltage that is supplied by the amplifier regardless of the input voltage, hence the one-to-one relationship between the output and the input is no longer valid. This results in a loss of information as the input cannot be inferred from the output waveform. To avoid such information loss, the output of the digital-to-analog converter is scaled with a coefficient that is less than one prior to feeding it to the power amplifier. This process is referred to as output back-off in the literature, and the coefficient to preserve the one-to-one relationship decreases as the PAPR of the waveform increases. As the waveform is scaled with a smaller coefficient, the average power decreases resulting in reduced signal power at the output. Furthermore, the relationship between the input and the output is also not linear, even in the linear region, for practical amplifiers. The output voltage is in fact a nonlinear function of the input voltage. This nonlinear relationship degrades the output signal, which decreases the signal to noise ratio (SNR) if the degradation is considered a noise, causes subcarriers of a multicarrier signal to interfere with one another, referred to as inter-carrier interference (ICI), and increases the OOB emission. Had the amplitude of the input voltage been constant at all times, it would have been scaled with the same coefficient and the output would not experience any of these problems. Therefore, high PAPR values degrade many aspects of the communication.
A method to reduce the PAPR and OOB emission of the OFDM waveform involves alignment signals. Alignment signals are designed to reduce the PAPR and OOB emission of the signal they are designed for when added to it and are also designed to “align” with the null space of the receiver pulse function upon convolution with the alignment filter. Thus, they minimize the problems that are experienced at the transmitter, and upon convolution with the alignment filter at the receiver, they disappear and do not cause problems to the receiver.
Current methods focus on windowing performed by extending the symbols by an amount which is arbitrarily determined, in addition to standard CP duration, wherein the focus is on deriving window functions optimized according to maximizing standard performance metrics. The currently proposed extensions reduce the symbol rate and change the frame structure defined in the standard, thus creating nonstandard signals that are not orthogonal to the symbols that aims to share the same numerology. This is not acceptable in the current cellular communication standards. Furthermore, extending the symbol duration relentlessly causes the symbol duration to exceed the coherence time of the channel, which is a critical problem for high-speed vehicular communications. Attempts have been made to improve spectral efficiency of windowed OFDM systems by utilizing less extension for inner subcarriers and assigning edge subcarriers to users with lower delay spread to use more windowing in edge subcarriers. However, this scheme still dos not comply with the standard frame structure. Additionally, standard compliant schemes have been derived for receiver windowing durations that optimize reception of each subcarrier in the case in which intersymbol interference (ISI) and ACI occur simultaneously and pulse shapes of transmitters operating in adjacent bands cannot be controlled, in the absence of any extension designated for windowing. However, determining whether it is more beneficial to window a duration at the transmitter or receives has not been previously addressed. Current methods focusing on alignment signals use part of the standard CP extension assuming it is not disrupted by anything else, yet again there's no study on the amount of CP to be assigned for alignment signals, especially in combination with transmit and receiver windowing, to optimize network conditions. Furthermore, PAPR and OOB emission are competing goals in the calculation of the alignment signals, and the earlier studies assign weights to each goal randomly. No study has been made to personalize the weights depending on the user's and the network's condition. The design of the filters themselves was not studied either.
Accordingly, what is needed in the art is an improved system and method that optimizes the combination of transmitter windowing, receiver windowing and alignment signals to maximize the overall capacity of the communication network.